Hydrofluorocarbons (HFCs), in particular hydrofluoroalkenes, such as tetrafluoropropenes (including 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf or 1234yf)) have been disclosed to be effective refrigerants, fire extinguishants, heat transfer media, propellants, foaming agents, blowing agents, gaseous dielectrics, sterilant carriers, polymerization media, particulate removal fluids, carrier fluids, buffing abrasive agents, displacement drying agents and power cycle working fluids. Unlike chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs), both of which potentially damage the Earth's ozone layer, HFCs do not contain chlorine and, thus, pose no threat to the ozone layer.
In addition to ozone depleting concerns, global warming is another environmental concern in many of these applications. Thus, there is a need for compositions that meet both low ozone depletion standards as well as having low global warming potentials. Certain fluoroolefins are believed to meet both goals. Thus, there is a need for manufacturing processes that provide halogenated hydrocarbons and fluoroolefins that contain no chlorine that also have a low global warming potential. One tetrafluoropropene having valuable properties is 2,3,3,3-tetrafluoropropene (HFO-1234yf). Thus, there is a need for new manufacturing processes for the production of tetrafluoropropenes and in particular 2,3,3,3-tetrafluoropropene.
HCFC-244bb is an intermediate in the production of 2,3,3,3-tetrafluoropropene (HFO-1234yf) which is well known in the art. U.S. Pat. No. 8,058,486, the contents of which are incorporated by reference, discloses a process of making HFO-1234yf starting with chlorinated hydrocarbons. The process has three-steps as follows:                (i) (CQ2=CCl—CH2Q or CQ3-CCl═CH2 or CQ3-CHCl—CH2Q)+HF→2-chloro-3,3,3-trifluoropropene (HCFO-1233xf or 1233xf)+HCl in a vapor phase reactor charged with a solid catalyst;        (ii) 2-chloro-3,3,3-trifluoropropene (HCFO-1233xf)+HF→2-chloro-1,1,1,2-tetrafluoropropane (HCFC-244bb or 244bb) in a liquid phase reactor charged with a liquid hydrofluorination catalyst; and        (iii) 2-chloro-1,1,1,2-tetrafluoropropane (HCFC-244bb)→2,3,3,3-tetrafluoropropene (HFO-1234yf) in a vapor phase reactor.wherein Q is independently selected from F, Cl, Br, and I, provided that at least one Q is not fluorine.        
The first step involves fluorination of tetrachloropropene or pentachloropropane with HF to produce HCFO-1233xf. The second step involves hydrofluorination of HCFO-1233xf with HF to produce HCFC-244bb. However, the conversion of HCFO-1233xf is not complete. Some of unreacted HCFO-1233xf is recycled back into the second step hydrofluorination reactor, but some of HCFO-1233xf is carried forward into the third step dehydro-chlorination reactor. The third and final step involves dehydrochlorination of HCFC-244bb to produce HFO-1234yf product. Again, conversion of HCFC-244bb is not complete. Unreacted HCFC-244bb and HCFO-1233xf carried from the third step reactor are recycled back to the second step reactor. But, the presence of HCFO-1233xf in the third step reactor feed does not allow recycle of all unreacted HCFC-244bb to the third step reactor. This results in larger size (lower capacity) of the second step reactor. Also, recycle of HCFC-244bb back into the second step hydrofluorination reactor may result in the formation of over fluorinated by-products such as 1,1,1,2,2-pentafluoropropane (HFC-245cb) and increased HF consumption.
It would be preferred to remove HCFO-1233xf and other halogenated olefins impurities produced in the first two process steps from the HCFC-244bb intermediate product prior to sending the feed into the dehydrochlorination reactor to produce final product HFO-1234yf. This would allow recycle of all unreacted HCFC-244bb back to the third step reactor minimizing the yield loss.
Unfortunately, HCFC-244bb and HCFO-1233xf are inseparable using conventional separation techniques known in the art since HCFC-244bb and HCFO-1233xf form a binary azeotrope or azeotrope-like composition which is described in U.S. Pat. No. 7,803,283. Since the boiling points of 1233xf and 244bb are only about 2° C. apart, separation of them is difficult and expensive.
Moreover, the presence of 2-chloro-3,3,3-trifluoropropene (HCFO-1233xf) in the reaction starting materials, such as HCFC-244bb feedstock, can lead to dramatically reduced conversion of HCFC-244bb to HFO-1234yf as well as increased formation of undesired trifluoropropyne (CF3CCH) byproduct through its dehydrochlorination. In addition, the 2-chloro-3,3,3-trifluoropropene copresence in the starting material, when subjected to dehydrochlorination, can lead to the formation of oligomers, which can produce tar. This result is disadvantageous from the standpoint of a reduced yield of the desired product.
One technique to remove 1233xf from 244bb is described in US 2013/0085308, the contents of which are incorporated herein by reference, which employs activated carbon as an adsorbent. US2013/0085308 additionally reports, at Example 4, that a molecular sieve of 4 Å pore size was unsuccessful in separating 1233xf and 244bb. Regenerating the activated carbon is economically important. US 2013/0085308 discloses that the activated carbon used to separate 1233xf from 244bb can be regenerated by heating, vacuum or an inert gas stream. Nevertheless, there is a need for other techniques to separate 1233xf and 244bb, and a need for other adsorbents and other methods of regenerating these.
The present invention fulfills that need.